Package update on-page notification
It would be a good idea to display an on-page notification on the "Edit a Package" page, to let administrators, as well as resellers know that changes will be applied to all accounts that have the edited package assigned. This could be something like:
The changes you've made will be applied to all accounts that have "package_name" assigned!
This could be taken a bit further by displaying a list of accounts that would be affected by the changes made in that particular package.
Furthermore, there could be checkboxes for each account in order to let the administrator chose the account/accounts that will be modified. The unchecked accounts would remain unchanged, perhaps by cloning the old package ("package_name_old" or "package_name_YYYY_MM_DD") and assigning it to them.